Some pneumatic devices, such as air logic system components, require only low flows for their operation but need pressures above 35 psi to operate reliably. In some instances where it is desirable to use these devices in conjunction with very low pressure systems, there is a need for a low pressure system to effect control of the high pressure system. For example, this would be true in a ventilator/resuscitator such as the type shown in U.S. Pat. No. 4,651,731, the subject matter of which is incorporated herein by reference thereto. Thus, in a ventilator/resuscitator the system may normally be cycled by high pressure air (for example 50 psi) under primary control. However, it may be desirable to provide patient override control means having input pressures plus 10 to 70 or minus 1 to 3 cm. of water. In order to provide satisfactory operation of such air logic control valves it is therefore necessary to have a relatively high work ratio where a pressure of one unit pressure could control, for example, a pressure approximately 1500 times greater.
While prior art devices for this purpose are known in the art, (such devices customarily being designated as "demand valves"), they are generally large, expensive, and can be easily damaged. In addition, they customarily operate in only one mode, such as only under the influence of negative pressure. Typical demand valves are shown in U.S. Pat. Nos. 2,860,631, 3,595,226, 4,416,176, 4,334,532 and 4,378,011.